Mapping or 3D-imaging of small objects such as teeth or fossils and artifacts and the like is important to enable obtainment of constructional data which is necessary for special analysis and computer-controlled manufacture of a replacement such as a tooth replacement as an example.
Quantitative measurement systems are traditionally classified according to the principle by which data are collected, such as contact or non-contact, surface topography or silhouette tracing. However, with the advent of digital technology, whatever the method of data collection, data are reduced to 2D or 3D coordinate data. The data can then be manipulated depending on the requirements for imaging or further processing and analysis.
Coordinate points can be taken as single predetermined points or at random, a collection of points along a profile, along contours and/or around image outlines. So, for example, where a silhouette of an object is hand traced, photographed or captured with a video camera, the silhouette line is sampled at regular intervals to extract 2D coordinates. The third coordinate can be derived from the position of the silhouette within the object. This method may require destruction of the object or replica being measured.
Laser-stripe scanning/profiling results in the recording of the stripe digitally. The stripe is then sampled or broken up into its component parts to extract 2D data. Once again, the stacking of many stripes leads to the determination of 3D coordinates. While the two techniques may appear quite different, basic principles remain much the same. Quantitative determination of tooth and lesion geometry has been performed using variations on these principles with varying accuracies and precisions, and have been performed directly on teeth in the mouth, on elastomeric impression negatives, or on positive replicas of teeth poured from elastomeric impressions.
Laboratory based profilometry of tooth replicas with contact stylus systems has been the norm since the early 1980s. Precisions in the order of a few micrometers have been commonly reported by the middle 1990s (e.g. Lee I K, DeLong R, Pintado M R, Malik R. Evaluation of factors affecting the accuracy of impressions using quantitative surface analysis. Operative Dent. 1995; 20:246-52.) However in order to gain high accuracy only one tooth can be profiled at a time, the surface of both tooth replica and stylus tip degrade with time leading to reduced accuracy. Tooth replicas are specially prepared and mounted on a rotating turntable. Full measurement of the geometry of a tooth may take several hours. Contact stylus systems are available for the measurement of larger objects comprising full dental arches however they yield lower accuracy and are unable to measure surface points at the steep angles leading to areas between teeth.
Laser scanning/profiling of replicas has become more common since the late 1990s. Precision in the range of 1 to 1O μm have been reported (e.g. Mehl A, Gloger W, Kunzelmann K-h, Hickel. R. A New Method of 3D Device for Detection of Wear. J. Dent.-Res. 1997; 76(11): 1799-807.). This method is similar to contact stylus profiling with the laser stripe replacing the contact stylus. It has the significant advantage of being very fast, with surface topography being recorded in a matter of seconds to minutes. However this “method typically requires the preparation of individual tooth replicas mounted onto a rotating turntable.
An alternative field to profilometry is that of machine vision where the projection of multiple stripes onto a surface and recording the scheme on one image. Machine vision involves three primary components—a laser light source, video camera and a computer. Typically a laser light grid is projected onto a light strip or planar substrate parallel to the plane of a sensor/CCD/film and the resulting light pattern is recorded as a template. The light pattern is then projected onto a scene. Analysis of the difference/distortion of the light pattern between the template and scene is analyzed by computer software to quantify the 3D profile of x, y and z co-ordinates of each point of the projected scene. There are however limits to the resolution of the light pattern that can be projected, leading to limitations on the grid density that can be determined. This approach is for practical purposes best suited to machined objects consisting of planes, circles and arcs for which only a small number of points is required to mathematically describe the geometry of that part of the surface. Boundaries between geometric surfaces can be easily and accurately interpolated. An application of this is quality control in the manufacture of aeroplane wings, where the entire assembled wing is wheeled into a hanger and photographed with a dozen cameras from different angles, instantaneously, and the geometry of the wing checked against a template. This type of technique has not been considered adequate for mapping of biological surfaces due to their irregular nature and consequent need for dense point determination.
Machine vision using direct optical 3-D surveying techniques have been applied in clinical practice on tooth replicas (models formed from an impression negative) and directly in the mouth of a patient where the tooth surface has been machined/drilled to regular shape in preparation to receive a machined restoration/filling. A French system based on the work of physician Dr. Duret is known to operate with a laser-triangulation method for the point-by-point measurement of the distance between a tooth surface and an optical probe, in which the optical probe is inserted into the oral cavity of the patient. By carrying out either a point-by-point distance measurement or through scanning by projecting a laser along a line, relative height coordinates of a scanned object can be obtained along the scanning line. CCD-scanning line sensors are ordinarily employed as optical pick-ups or receivers enabling pick up of point rasters.
A Swiss system utilized by the company of BRAINS, Brandistini Instruments, Switzerland, designated by the description CEREC, operate. in accordance with a light-section technique in which a single line or dash of light or parallel grid consisting of dashes or lines of light are projected onto a surface and observed under a parallax-angle with a two-dimensional camera. From the curvature of lines of the light-section relative heights can be computed. A variant of this technique referred to as the ‘phase-shift’ method is known which employs an interferometrically-produced light grid with sinusoidal brightness modulation in contrast with the binary light-sections. Through a pick-up or recording of an object at a plurality of positions for the phase location of this grid, there can be obtained in a significantly higher point density of height values and any disturbing influences, such as non-constant background brightness and fluctuating stripe or line contrast caused by localized fluctuations in reflection, which can be mathematically eliminated.
The structured light technique utilized by the CEREC system is the only intra-oral method currently available for tooth mapping. It was specifically designed for the mapping of prepared tooth cavities and has not been used for general tooth mapping. Its utility in this regard is unknown. Its reported accuracy of 25 μm for mapping and 40 μm error associated with the need to apply an opaque powder to the tooth surface intra-orally is inferior to that of laser or contact mapping and would appear inadequate for the monitoring of tooth wear. However, as a direct method, it is considerably faster and more convenient than other methods.
A further system proposed by Massen; Robert (Radolfzell, D E); Gassier; Joachim (Constance, D E), U.S. Pat. No. 5,372,502 is an optical three-dimensional measuring probe which is utilized to generate a three-dimensional image of a single tooth or a group of teeth within the oral cavity of a patient. The measuring probe projects a particular pattern onto the single tooth or group of teeth which is/are to be surveyed. The particular pattern projected can be, for example, a series of parallel stripes. This projected pattern of stripes is distorted by the tooth or teeth which is/are to be measured due to variations in height. Basically, the pattern is distorted by the tooth or teeth which is/are to be measured in that the individual stripes fall on sections of the tooth which are of different height or fall on different teeth which are different height. The distorted pattern is reflected back towards the measuring probe, which captures the distorted pattern and transmits it ultimately to a computer. Through a comparison between the undistorted pattern projected by the probe and the distorted pattern reflected from the specific area within the oral cavity, information with respect to the topography of the tooth or teeth is obtained. In order to preclude ambiguities in this topographical information and to increase the precision of the measurement, the surveying procedure is repeated a number of times whereby the pattern, which is projected against the tooth or teeth, is always varied. Accordingly, the distorted pattern, which is captured by the measuring probe, will also vary; however, each iteration provides refinement of the topography. This approach describes a further refinement to the Cerec system. The system may yield some technical improvements however they would appear to be of limited practicality.
A literature review of the techniques to measure tooth wear and erosion (Azzopardi A, Bartlett D W, Watson T M, Smith G N. A Literature Review of the Techniques to Measure Tooth Wear and Erosion. Eur. J. Prosthodont. Rest. Dent. 2000; 8(3):93-97) concluded that profilometry remained a technique limited to the laboratory and that there was a need for a simple, reliable technique. No technique has been used sufficiently extensively clinically to merit widespread application.
During the 1970's and early 1980's, an alternative approach using photogrammetry techniques was investigated by several authors and produced accuracy of 10 μm in limited laboratory studies (Clarke C E, Flinn R M, Atkinson K B, Wickens E H. The measurement Comparison of Tooth Shape Using Photogrammetry; Photogrammetric Record 1974; 8(44):217-21; Chiat B., The shapes of small pebbles; Photogrammetric Record 1977; 9(49):77-82. Adams L P. The use of a non-metric camera for very short-range dental stereophotogrammetry. Photogrammetric, Record 1978; 9(51):405-14; Lamb RD, McGarrah H E, Eick J D. Close-range photogrammetry with computer interface in dental research. Photogrammetric Engineering and Remote Sensing 1987; 53(12):1685-89.).
The majority of work on teeth has been conducted with microscopes, however the applicant has come to realize that work on the cornea with macro-lens cameras (Osborn J E. Stereophotogrammetric mapping of the anterior surface of the human cornea. Int Arch photogram and remote sensing 1996; 31 (Part B5):443-49) may be suited to imaging teeth both clinically and in the laboratory on replicas. While photogrammetry has the advantage of capturing images quickly for later processing at a convenient time, Clarke et al (1974) noted that an experienced operator could record 1000 points in 4 hours. This very slow recording time for surface measurement when compared with modern laser scanning where many thousands of points can be recorded in a matter of seconds has rendered photogrammetric method impractical for high-resolution measurement of small objects.
Photogrammetry applies techniques, which are used and were principally developed for land mapping based on taking measurements off aerial photographs. Two kinds of photograph are used in photogrammetry, aerial and terrestrial. In aerial photogrammetry a sensor location (camera) is “remote” (in an aeroplane) from an object or scene. In this application there is a need for the calculation of a large number of unknown parameters in order to build an accurate model of the terrain below. Photogrammetry relies on the presence of sufficient natural features on the surface of a scene to perform triangulation and height determination. It has the principal advantage of fast and convenient image acquisition using relatively inexpensive camera equipment with the possibility of images being processed with photogrammetric workstation software at a later time when a topographical map is required. So images can be recorded and stored for years if necessary before photogrammetric processing is performed and high quality topographic measurement is performed. Alternatively, a calibrated stereometric camera and automated software that performs establishment of corresponding points in the images (Image Matching), computation of their 3D coordinates; and generation of a surface model, may be used to generate a topographical map within a matter of seconds. Many attempts to extract 3D coordinated data from small objects, especially teeth, have been limited to extracting 3D coordinated data from the outlines of particular features and have focused on the theoretical accuracy and precision that can be achieved, but have fallen well short in applicability due to the lack of natural features present on the entire surface. This characteristic is referred to as “optical texture”.
There has been the advent of higher resolution digital cameras and automatic processing (e.g. Mitchell H L, Kniest H T, Oh W J. Digital photogrammetry and microscope photographs. Photogrammetric Record, 1999; 16(94):695-704.). It could be presumed that this makes developing the photogrammetric approach to laboratory and/or intra-oral mapping of teeth a real possibility. However, an attempt by Mitchell et al to use digital photogrammetry to map a tooth replica was unsuccessful due to a lack of radiometric (optical) texture and to date there are no reports of successful stereo photogrammetric mapping of complete tooth surfaces, either natural, impression negatives or replicas.
Materials which are frequently used for impression negatives and in particular replicas of small objects fall into three broad categories: mineral e.g. gypsum products; polymers e.g. epoxy, urethane, styrene etc; and metals.
Where die materials are to be mapped with a mechanical probe profiler, the material must have sufficient rigidity to resist deformation and excessive abrasion during contact; type IV diestones and metals are typically used for this purpose. Where die materials are mapped with a non-contact laser profiler or structured light projection system, the material must be sufficiently optically dense so that light is reflected from the surface of the die. This is not a problem with gypsum products, but polymers, which may be naturally clear, must be coloured sufficiently densely so that no light is reflected from below the surface resulting in excessive light scatter. Recently 3M has produced an experimental polyvinyl siloxane elastomeric material known as ‘Digisil’ which has been coloured with the aim of improving the surface reflection of laser profilometry equipment (DeLong R, Heinzen M, Hodges J S, Ko C C, Douglas W H. Accuracy of a system for creating 3D computer models of dental arches. J Dent Res. 2003; 82(6):438-42.). The application of metal and paint films to replicas to enhance their surface properties in conjunction with contact stylus profiling has also been reported (Chadwick R G, Mitchell H L, Ward S. Evaluation of the accuracy and reproducibility of a replication technique for the manufacture of electroconductive replicas for use in qualitative clinical dental wear studies. J. Oral Rehabil. 2002; 29:540-45.). Die or model materials are known in the diagnosis and treatment of a dental condition. Indeed harden-able polymeric materials are used in a number of dental applications comprising composites, filling materials, restorations, cements, and adhesives. To date there appears to be no successful 3D-imaging of small objects by stereo photogrammetric mapping of objects from conventional die materials.
One recent attempt to provide images of small mammalian teeth has used a method in which a replica is created by mixing a fluorescent dye in a urethane polymer and imaging the replica with a confocal microscope. This method is however slow and requires the use of very expensive confocal imaging equipment (Evans A R, Harper I S, Sanson G D. Confocal imaging, visualisation and 3-D surface measurement of small mammalian teeth. J. Microsc. 2001; 204(2):108-19.). Typically several images at lower magnifications need to be combined to map human teeth, with a slight reduction in accuracy. Computed tomography is in an early phase of study. Its future utility is uncertain.
It is to be understood that any discussion of prior art heretofore is not an admission that such art constitutes common general knowledge.
The invention aims to improve optical texture characteristics to allow structural and topographical mapping by photogrammetry.
The present invention therefore is to provide a composition with improved optical texture to allow imaging of impressions or replicas of small objects by photogrammetry.
A further object is to provide an alternative to existing techniques of imaging objects, which ameliorates one or more of the disadvantages of the prior art.